CTest: convert timeouts to std::chrono::duration

This commit continues the refactoring of CTest to adopt std::chrono.
After the last sets of changes that introduced std::chrono::steady_clock
and std::chrono::system_clock respectively, it makes sense to have all
the timeouts be stored as std::chrono::duration.

No functional change intended.

Source/CTest/cmCTestScriptHandler.h

@@ -94,9 +94,9 @@ public:
   /**
    * Return the time remaianing that the script is allowed to run in
    * seconds if the user has set the variable CTEST_TIME_LIMIT. If that has
-   * not been set it returns 1e7 seconds
+   * not been set it returns a very large value.
    */
-  double GetRemainingTimeAllowed();
+  std::chrono::duration<double> GetRemainingTimeAllowed();
 
   cmCTestScriptHandler();
   ~cmCTestScriptHandler() override;